For pain to be perceived there is usually a chain whereby peripheral receptors are stimulated by a noxious physical or chemical agent and this stimulus is carried by peripheral nerves to the spinal cord, up to the cord, through the brainstem and so to the cerebral cortex, where the pain is appreciated by the conscious level. This route necessarily involves a number of synapses and the inhibition of impulses on their route to the cortex is the mechanism whereby pain can be modulated.
Nociceptive nerve endings may be stimulated by the chemicals released by tissue injury or accumulated as a result of metabolic activity, and thus creating an electrical potential. The degree of stimulation produced is governed by the amount of chemicals present. It is postulated that the removal of these chemicals from the area may help reduce the level of nociceptive stimulation and thus the physiotherapeutic agents (ice and heat) affecting the circulation may help achieve this. The nociceptive stimulus is carried to the cord along with either a slow-conducing, non-myelinated C fiber or a faster myelinated Ad (delta) fiber. Both will eventually enter the cord via the posterior route. It is postulated that as both of these fibers have a maximum frequency at which they can conduct that if a higher frequency of stimulation is applied, a physiological block o conduction might occur. TENS produces this required frequency and so has this effect
Pulse shape - Is usually rectangular.
Pulse width - is measured in microseconds and is between 100 and 500us.
Frequency - can be as low as 2 Hz or as high as 60Hz. A frequency of 150 Hz is commonly used. The frequency can be selected and altered by the therapist.
Intensity – can be varied from 0 to 60 milliamps (m A) on many units. The patient or therapist can control the intensity and a tingling sensation should be felt.
The wide range of variation in pulse width, frequency and intensity gives flexibility in terms of the treatments applied to patients with chronic pain syndromes.
Vacuum electrodes or rubber electrodes are strapped or fixed with the adhesive tape and covered with a conductive gel and then placed on the patient’s skin. The wires connecting the electrodes to the unit can be concealed by clothing.
Positioning of electrodes
A number of approaches may be used. Electrodes can be placed over
· Acupuncture points, motor points or trigger points.
· Area of greatest intensity of pain
· Appropriate dermatome or spinal segment
· Appropriate peripheral nerve
Whichever position is chosen for electrode placement, it is best if the skin below them has an intact sensory mechanism as it is the large diameter afferent sensory stimulation produced by the TNS current acting on the skin that produces the effect on pain. Once the electrodes have been positioned , the TENS can be applied by setting the parameters.